Some autogyro aircraft have jump takeoff ability, in which the rotor is powered on the ground to a high rotation rate at zero rotor blade pitch (called prerotation), then the pitch is increased for vertical takeoff. The rotor is disengaged from the engine prior to takeoff. Such aircraft require a clutch between the rotor and the engine so that engine power can be disconnected from the rotor prior to takeoff. The rotor must not be powered in flight because autogyros do not have a tail rotor to counteract the torque applied to the main rotor.
In addition, piston engine autogyros require a clutch so that the engine can be started without the need to turn the rotor at the same time. To engage the clutch without stalling the engine, the clutch must have the ability to limit torque applied to the rotor. Rotary wing aircraft rotors turn at relatively low speeds compared to aircraft engines, requiring a reduction unit to be introduced between the engine and the rotor.
In autogyros, the prerotation mechanism, reduction drive, and rotor drive shaft can be engineered to be very light since they are powered only prior to takeoff (non-continuous) and at a torque level no greater than needed to adequately accelerate the rotor and overcome rotor profile drag (low horsepower relative to available engine horsepower). The prerotation mechanism thus requires a maximum torque limiting device to avoid overstressing the lightweight, light duty prerotation mechanism.